Temperature measurement by a temperature measuring resistor element is performed using a property whereby an electric resistance of a temperature measuring resistor wire housed in an interior of the temperature measuring resistor element varies according to a temperature such that the temperature of the temperature measuring resistor wire serves as a temperature output of the temperature measuring resistor element. More specifically, a specified current is passed through lead wires connected to respective ends of the temperature measuring resistor wire, an electric resistance value is obtained from a voltage drop, and the electric resistance value is converted into a temperature. As is well known, a voltage drop measurement method includes a two-wire system, a three-wire system, and a four-wire system, depending on the number of bifurcations of the lead wire.
In a usually employed configuration of a temperature measuring resistor element, a coil-shaped temperature measuring resistor wire is disposed in inorganic electric insulating powder accommodated in a housing.
This configuration is usually employed in a temperature measuring resistor element because of the easy manufacture and the low cost. Another reason why this configuration is typically employed is that when an external force is exerted on the temperature measuring resistor wire due to a difference in the thermal expansion coefficient between the temperature measuring resistor wire and a supporting structure thereof, the length and the sectional area of the temperature measuring resistor wire vary, leading to a variation in the electric resistance and then a temperature measurement error, but in the configuration where the coil-shaped temperature measuring resistor wire is disposed in the inorganic electric insulating powder, little force is exerted on the temperature measuring resistor wire, and therefore a temperature measurement error is unlikely to occur.
In a temperature measuring resistor element, a coil-shaped temperature measuring resistor wire, to the opposite ends of which lead wires are connected, is passed through two longitudinal through holes formed in a cylindrical electric insulator having a substantially circular cross-section, and serving as an housing. The two lead wires connected to the opposite ends of the resistor wire are exposed from end portions of the two through holes, the end portion being on the same side. The temperature measuring resistor wire is disposed in filler in the through holes, the filler made of an inorganic electric insulating powder being filled into gaps in the through holes, and the opposite end portions of the through holes are sealed by sealers to prevent the internally placed filler from falling out.
Polycrystalline powder made of such material as alumina, magnesia, silica, or a mixture of these materials is used as the inorganic electric insulating powder serving as the filler. A ceramic formed by molding a polycrystal made of such material as alumina, magnesia, silica, a mixture of these materials is used as the electric insulator.
A polycrystalline adhesive having alumina, magnesia, silica, zircon, or a mixture of these materials as a main component is usually used as the sealer, and an enamel such as epoxy resin may also be used. Further, as shown in FIG. 1A of Patent Document 2, the sealer is not provided in a temperature measuring resistor element used in a condition where there is no possibility that inorganic electric insulating powder falls out.
As described in JISC1604 “Temperature Measuring Resistors”, platinum is usually used as a material for the temperature measuring resistor wire. However, when a measured temperature falls to or below 73 K (−200° C.), a variation rate in the electric resistance of the platinum relative to a temperature variation decreases, leading to a lowering of the measurement sensitivity, and therefore an increase in the temperature measurement error, making measurement impossible. Hence, when temperatures of 73 K or less are also subject to measurement, a platinum-cobalt alloy, as described in Patent Document 1 and Non-Patent Document 1, is usually used as the material for the temperature measuring resistor wire. With a platinum-cobalt alloy, the measurement sensitivity lowers small even in a low temperature region, and therefore temperature measurement can be performed with a high degree of precision in a temperature range of 4 K to at least 325 K, as illustrated in Non-Patent Document 1. The reason why 4 K is a lower limit is that 4 K is the boiling point of helium, and at temperatures no higher than 4 K, temperature calibration is difficult.